The present invention relates to a two-track convolver. A two-track convolver having suppression of self-convolution signals is known from Proc. IEEE, Ultrasonics Symposium (1974), pages 224-227 and (1981), pages 181-185.
The acoustic waves employed in conjunction with convolvers and similar electrical arrangements involve acoustic waves which proceed in a substrate close to the surface or in the surface. Such acoustic waves are known as Rayleigh waves, Bleustein waves, Love waves, SSBW waves, SABS waves, and the like which shall be referred to below in general as surface waves (even though only the first two wave types are essentially understood as surface waves in the narrowest sense).
A surface wave convolver is an electrical means for extremely high frequencies, particularly beginning in the MHz region. Such a convolver is employed for the processing of, for example, binary orthogonal keying (BOK) signals. A convolver is a compilation of a plurality of structures arranged on the surface of a substrate, and composed, for example, of lithium niobate. Structures for the two-track construction of the convolver employed for self-convolution suppression include two strip-shaped integration electrodes, two respective beam compressor structures added thereto under given conditions, and two respective, i.e. a total of four input transducers. Two input transducers are intended for the input signals E, and two input transducers are intended for the reference signal (R). The output terminals of the two integration electrodes are electrically connected to one another via a repeater or transformer. The repeater or transformer is the actual output of the two-track convolver. The processed input signals corresponding to the function of the convolver can be obtained at this output without having the self-convolution signals appearing as well.
The self-convolution signal is based on an acoustic wave generated in the input transducer for the reference signals by the acoustic wave sent into the convolver from the input transducer of the input signal. This acoustic wave runs in the opposite direction in the region of the integration electrode and generates self-convolution signals (which are undesired) together with the wave still being supplied by the input signal of the input transducer.
The suppression of the self-convolution is possible since, as is known, these respectively returning waves are antiphase relative to one another in the two integration electrodes.
As is the case for all electrical arrangements of high-frequency technology, care must be exercised regarding the proper matching of the individual, existing networks. For a two-track convolver as well, a respective matching network is required for the side of the input signal and for the side of the reference signal in order to have matching of the respective input impedance. The simplest possible matching network is an inductance whose value of inductance is matched to the capacitance of the respective input transducer. In comparison to one-track convolvers, however, two respective input transducers are connected in parallel in a two-track convolver. This is advantageous in order to guarantee high electrical symmetry of the two respectively coupled input transducers. As a result of the parallel connection, however, an impedance that is equal to half the impedance of every individual input transducer is obtained at the terminals of these transducers.